The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the claimed invention.
Lithium ion batteries have been proven to offer higher energy and power density, a wider range of operating temperatures, and excellent cycle and calendar life when compared to other battery chemistries. Continued demand for various portable electronics, such as electric hand and power tools, as well as high power applications of electric based transportation, continues to direct research to focus on lower cost materials without compromise of reliability and life of lithium ion batteries. As a result, the lithium-sulfur cell has become an attractive option because of the high theoretical specific energy of about 2600 Wh/kg (1672 mAh/g), assuming complete reaction to Li2S.
Elemental sulfur, however, poses two problems when used as a cathode active material in a lithium-sulfur cell. First, sulfur itself has a very low electrical conductivity; for example, about 5.0×10−14 S cm−1 at 25° C. Second, sulfur has a high solubility in the electrolyte of a cell during lithiation, or discharge. Dissolution of sulfur during charging and discharging reduces the capacity of an electrochemical cell, and is not preferred. For example, after dissolution, the sulfur anions then re-precipitate and react on a carbon anode surface. Accordingly, there remains a need for lithium-sulfur cell batteries having improved energy and power output. The present disclosure provides a new sulfur-containing cathode material that minimizes the current problems associated with the high resistivity, dissolution, and reactivity of sulfur, while maintaining a desired useable life. There have been many attempts in the past to compound elemental sulfur with carbon particulate to enhance electrical conductivity. In addition, recently work has been performed to trap the sulfur in micro and meso pores of carbon particulates. However, deposition of sulfur in these types of carbon did not prevent exposure of the sulfur to the electrolyte, and only provided limited cycle life with fast capacity decay. This invention provides a process and method to encapsulate sulfur in the hollow core of carbon nanofiber with high aspect ratios.